A known technique assembles two members by engaging the two members with each other. Japanese Unexamined Patent Application Publication No. 9-83004 discloses a technique that assembles a cover and a box. Japanese Examined Utility Model Registration Application Publication No. 6-20301 discloses a technique that assembles a plug and a receptacle. Also, a known technique includes an elastic member pinched between first and second casings when the first and second casings are assembled to each other, to inhibit water from entering the space defined by the first and second casings. The first casing includes a pressed portion that is pressed by the elastic member.